Some printed matter such as newsprint or product labels can be printed in a single layer. An example, newsprint, is usually printed in one layer of a single ink (black). In more advanced systems, some information must be printed on top, overlaid, onto a previously printed layer. Color photographs are printed in four layers, each with a different color ink. Each layer must be accurately overlaid in position onto the previous layer for the image to appear correct. Each layer has reference marks outside the area of information, (i.e. photo). The reference marks can be crosshairs, bullseyes, etc. The marks on the printing mask are aligned to some reference on the printing machine. The next layer, with identically located marks can be aligned to the same reference on the machine and information of the two layers will be correctly registered, or aligned. For a system like color photograph printing, alignment of crosshairs by an unaided eye is sufficient for proper registration. The accuracy required depends on how the information printed will be used. For color photographs, it is intended to be viewed by an unaided human eye at a comfortable reading distance. It is a simple test to find that the registration accuracy is not very precise in such lithographic systems. Anyone who has used a magnifying glass to view such printed photos will recall seeing the colored dots that comprise the photo simply near each other but not aligned in an orderly array.
Lithographic systems for other applications sometimes demand far greater accuracy. An example is electronic circuit boards. The signal traces or electronic transmission paths can be very small. A device that uses multilayer paths will require that the paths of the second level be printed onto the paths of the first level such that the paths are positioned accurately with respect to each other. The width of the transmission path usually dictates the accuracy that the layers must be placed. For electronic applications it is common that the overlay accuracy be one fifth of the signal path width. Very advanced systems that are used to print microelectronics circuits require extremely small tolerances. A modern microelectronics circuit could have electronic paths as narrow as five microns, therefore requiring an overlay of one micron.
Currently holographic lithography systems use blind positioning methods for alignment. Wafer handling machinery can replace a wafer to a position with an accuracy that is a fraction of a millimeter without measurement and feedback means. Blind positioning can only be used for systems that can tolerate very coarse alignment.